Dual underground tunnel system for hydrocarbon exploitation

ABSTRACT

A system for recovering hydrocarbons from a subterranean formation includes an upper tunnel extending through the formation. In addition, the system includes a lower tunnel extending through the formation below a portion of the upper tunnel. Further, the system includes a plurality of conduits extending from the upper tunnel through the formation to the lower tunnel. Still further, the system includes a plurality of wellbores extending from the lower tunnel into a hydrocarbon-bearing portion of the formation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. provisional patent application Ser. No. 61/784,327, filed Mar. 14, 2013, and entitled “Dual Underground Tunnel System for Hydrocarbon Exploitation,” which is hereby incorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

The embodiments described herein relate generally to systems and methods for accessing and producing subsurface hydrocarbons. More particularly, embodiments described herein relate to systems and methods for exploiting hydrocarbons with a minimal surface footprint using dual underground access tunnels.

In drilling a borehole (or wellbore) into the earth for the recovery of hydrocarbons from a subsurface formation, it is conventional practice to connect a drill bit to the lower end of a drill string, then rotate the drill bit with weight-on-bit (WOB) applied to enable the bit to progress downward into the earth to create the desired borehole. A typical drillstring usually includes drill pipe sections connected end-to-end and a bottom hole assembly (BHA) between the drill bit and the lower end of the drill string. The BHA is typically suited to the requirements of the well being drilled and may include subcomponents such as drill collars, reamers, stabilizers, mud motor, or other drilling tools and accessories. In general, the drill bit can be rotated from the surface with a top drive or rotary table and/or rotated with a mud motor disposed in the drillstring. During drilling operations, drilling fluid or mud is pumped from the surface down the drillstring and out nozzles in the face of the drill bit. The drilling fluid returns to the surface via the annulus disposed between the drill string and the sidewall of the borehole. The drilling fluid carries borehole cuttings to the surface, cools the drill bit, and forms a protective cake on the borehole wall (to stabilize and seal the borehole wall), as well as other beneficial functions. At the surface, the drilling fluid is cleaned and conditioned (e.g., by removing borehole cuttings, adjusting the chemical composition, etc.), then re-circulated by pumping it downhole under pressure through the drill string.

Heavy oil reservoirs provide relatively new and untapped sources of hydrocarbons. Heavy oil deposits typically require a relatively high well density, on the order of tens to hundreds, to achieve economical levels of production and to provide an acceptable recovery factor. In some locations, this high density of wells may not be acceptable because of environmental impacts or other land use constraints.

BRIEF SUMMARY OF THE DISCLOSURE

These and other needs in the art are addressed in one embodiment by a system for recovering hydrocarbons from a reservoir contained in a subterranean formation. In an embodiment, the system comprises an upper tunnel extending through the formation. In addition, the system includes a lower tunnel extending through the formation below a portion of the upper tunnel. Further, the system includes a plurality of conduits extending from the upper tunnel through the formation to the lower tunnel. Still further, the system includes a plurality of wellbores extending from the lower tunnel to the reservoir in the formation.

These and other needs in the art are addressed in another embodiment by a method for recovering hydrocarbons from a reservoir in a subterranean formation. In an embodiment, the method comprises constructing an upper tunnel that extends through the formation. In addition, the method comprises constructing a lower tunnel that extends through the formation and is disposed below a portion of the upper tunnel. Further, the method comprises drilling downward from the upper tunnel through the lower tunnel and into the formation toward the reservoir.

These and other needs in the art are addressed in another embodiment by a method for recovering hydrocarbons from a reservoir in a subterranean formation. In an embodiment, the method comprises (a) positioning a drilling rig at a first location in an upper tunnel traversing the formation. In addition, the method comprises (b) positioning a BOP at a first location in the lower tunnel that is below the first location in the upper tunnel. Further, the method comprises (c) advancing a drillstring from the drilling rig through the BOP. Still further, the method comprises (d) drilling a first wellbore downward from the lower tunnel with the drilling rig and drillstring.

Embodiments described herein comprise a combination of features and advantages intended to address various shortcomings associated with certain prior devices, systems, and methods. The foregoing has outlined rather broadly the features and technical advantages of the invention such that the detailed description of the invention that follows may be better understood. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the preferred embodiment of the invention, reference will now be made to the accompanying drawings in which:

FIG. 1 is a perspective view of an embodiment of a subsurface system for accessing and producing hydrocarbons in accordance with the principles described herein;

FIG. 2 is an enlarged perspective view of section 2-2 of FIG. 1;

FIG. 3 is a top view of the system of FIG. 1;

FIG. 4 is an enlarged side view of section 2-2 of FIG. 1;

FIG. 5 is an enlarged cross-sectional end view of section 5-5 of FIG. 4;

FIG. 6 is an enlarged side view of section 5-5 of FIG. 4;

FIG. 7 is an enlarged perspective view of the drilling rig of FIGS. 5 and 6;

FIG. 8 is a side view of the drilling rig of FIG. 7; and

FIG. 9 is a schematic view of a closed loop mud circulation system to circulate drilling fluid to the drill site of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following discussion is directed to various exemplary embodiments. However, one skilled in the art will understand that the examples disclosed herein have broad applications, and that the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosures, including the claims, is limited to that embodiment.

Certain terms are used throughout the following description and claim to refer to particular system components. This document does not intend to distinguish between components that differ in name but not function. Moreover, the drawing figures are not necessarily to scale. Certain features of the invention may be shown exaggerated in scale or in somewhat schematic form, and some details of conventional elements may not be shown in the interest of clarity and conciseness.

In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices, components, and connections. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a central axis (e.g., central axis of a body or a port), while the terms “radial” and “radially” generally mean perpendicular to the central axis. For instance, an axial distance refers to a distance measured along or parallel to the central axis, and a radial distance means a distance measured perpendicular to the central axis. Still further, reference to “up” or “down” may be made for purposes of description with “up,” “upper,” “upward,” or “above” meaning generally toward or closer to the surface of the earth, and with “down,” “lower,” “downward,” or “below” meaning generally away or further from the surface of the earth.

The use of an underground tunnel system in accordance with the present disclosure provides systems and methods for accessing hydrocarbons in harsh environments and/or environmentally sensitive geographic regions. In particular, such systems and methods enable recovery of hydrocarbon deposits such as conventional oil or heavy oil through one or more wellbores originating from an underground tunnel, thereby reducing the surface footprint of the associated drilling and production operations.

Referring now to FIGS. 1 and 2, an embodiment of a system 10 for drilling and/or producing hydrocarbons (e.g., conventional oil, gas, heavy oil, bitumen) from a subterranean formation 5 is shown. In this embodiment, system 10 includes an access tunnel 20 and a pair of operating tunnels 30, 40. As best shown in FIG. 2, system 10 includes one or more drill sites 15, each drill site 15 generally spans both tunnels 30, 40. Generally, the subterranean formation 5 comprises a hydrocarbon-bearing portion and a non-hydrocarbon-bearing portion with the access tunnel 20 and the operating tunnels 30, 40 disposed in the non-hydrocarbon-bearing portion and the drill sites 15 disposed in communication with the hydrocarbon-bearing portions of the formation 5.

Access tunnel 20 is primarily employed to deliver equipment and personnel to operating tunnels 30, 40 and drill site 15. Tunnels 30, 40 house drilling and production equipment (e.g., at site 15), and are primarily employed to perform drilling and production operations. As will be described in more detail below, access tunnel 20 is generally disposed below operating tunnels 30, 40, and tunnel 30 is positioned above tunnel 40. Thus, tunnel 30 may also be referred to as the “upper” operating tunnel 30 and tunnel 40 may also be referred to as the “lower” operating tunnel 40. As will be described in more detail below, operating tunnels 30, 40 are parallel, each extending along paths that contain predetermined drilling locations. Although tunnels 20, 30, 40 do not intersect, tunnel 20 is connected to each tunnel 30, 40 at various locations by a plurality of connection tunnels 50 (not shown in FIGS. 1 and 3), each tunnel 50 extending between the access tunnel 20 and one of the operating tunnels 30, 40.

Access tunnel 20 is a continuous subsurface tunnel that originates at the surface 6, slopes downward at an angle α₂₀ (measured downward from horizontal) to a depth between 300 and 3,000 ft. (measured vertically downward from the surface 6), and then extends horizontally through formation 5 below tunnels 30, 40. In addition, access tunnel 20 includes a plurality of parallel horizontal linear sections 21 interconnected with a plurality of horizontal U-turn sections 22 resulting in a geometry that generally winds back-and-forth in formation 5 within a horizontal plane. In general, access tunnel 20 can have any suitable size and geometry, however, in this embodiment, access tunnel 20 is generally cylindrical with a uniform diameter D₂₀ preferably between 20 and 60 ft. In this exemplary embodiment, diameter D₂₀ is 38 ft.

Referring still to FIGS. 1 and 2, upper operating tunnel 30 is a continuous subsurface tunnel that originates at the surface 6 proximal access tunnel 20, slopes downward at an angle α₃₀ (measured downward from horizontal) to a depth of between 200 and 3,000 ft. (measured vertically downward from the surface 6), and then extends horizontally through formation 5 above tunnels 20, 40. In addition, upper operating tunnel 30 includes a plurality of parallel horizontal linear sections 31 interconnected with a plurality of horizontal U-turn sections 32 resulting in a geometry that generally winds back-and-forth in formation 5 within a horizontal plane disposed above tunnels 20, 40. As best shown in FIG. 1, linear sections 31 of upper tunnel 30 are oriented perpendicular to linear sections 21 of access tunnel 20 in top view. In general, upper operating tunnel 30 can have any suitable size and geometry, however, in this embodiment, upper operating tunnel 30 is generally cylindrical with a uniform diameter D₃₀ preferably between 20 and 50 ft. In this exemplary embodiment, diameter D₃₀ is 30 ft.

Referring still to FIGS. 1 and 2, lower operating tunnel 40 is a continuous subsurface tunnel that originates at the surface 6 proximal tunnels 20, 30, slopes downward at an angle α₄₀ (measured downward from horizontal) to a depth of between 250 and 3,000 ft. (measured vertically downward from the surface 6), and then extends horizontally through formation 5 between tunnels 20, 30. In addition, lower operating tunnel 40 includes a plurality of parallel horizontal linear sections 41 interconnected with a plurality of horizontal U-turn sections 42 resulting in a geometry that generally winds back-and-forth in formation 5 within a horizontal plane disposed between tunnels 20, 30. As best shown in FIG. 1, linear sections 41 of lower tunnel 40 are oriented perpendicular to linear sections 21 of access tunnel 20 in top view. Further, linear sections 41 are oriented parallel to linear sections 31, and U-turn sections 42 are oriented parallel to U-turn sections 32 in top view. In general, lower operating tunnel 40 can have any suitable size and geometry, however, in this embodiment, lower operating tunnel 40 is generally cylindrical with a uniform diameter D₄₀ preferably between 15 and 40 ft. In this exemplary embodiment, diameter D₄₀ is 20 ft.

As previously described, the diameter D₃₀ of upper tunnel 30 is preferably 20-50 ft. and the diameter D₄₀ of lower tunnel 40 is preferably 14-40 ft. In addition, the ratio of the diameter D₄₀ of lower tunnel 30 to the diameter D₃₀ of upper tunnel 30 is preferably between 0.3 and 1.0, and more preferably between 0.5 and 0.75. In this embodiment, the ratio of the diameter D₃₀ of upper tunnel 30 to the diameter D₄₀ of lower tunnel 40 is 0.66.

As best shown in FIGS. 4 and 5, in this embodiment, access tunnel 20 is the deepest tunnel and is situated at vertical center-to-center distance D₂₀₋₃₀ below upper tunnel 30, and a vertical center-to-center distance D₂₀₋₄₀ below lower tunnel 40. Distance D₂₀₋₃₀ is preferably between 80 and 250 ft., and distance D₂₀₋₄₀ is preferably between 50 and 150 ft. In general, the actual distances D₂₀₋₃₀, D₂₀₋₄₀ will vary depending on a variety of factors including, without limitation, geology, faults, existing well bores, proposed well design, drilling equipment design, or combinations thereof. In this embodiment, distance D₂₀₋₃₀ is about 120 to 150 ft., and distance D₂₀₋₄₀ is 60 ft. Although access tunnel 20 is disposed below both operating tunnels 30, 40 in this embodiment, in other embodiments, the access tunnel (e.g., tunnel 20) is disposed above the operating tunnels (e.g., tunnels 30, 40).

As previously described, each tunnel 20, 30, 40 extends downward from the surface 6 at an angle α₂₀, α₃₀, α₄₀, respectively, preferably between 3° and 10°. In this embodiment, each tunnel 20 originates from a ground-level tunnel access 11 at surface 6. Accesses 11 are suitable for handling equipment employed in tunnels 20, 30, 40 as well as excavated tunnel materials. As will be described in more detail below, equipment, materials, personnel, or combinations thereof are transported through tunnels 20, 30, 40 on rail cars disposed on tracks, rail cars disposed on tracks and including auxiliary traction systems, rubber-tired transport vehicles, or combinations thereof. In general, rail cars are suited for portions of tunnels 20, 30, 40 having a grade less than 2°, whereas rubber-tired vehicles and railed systems with auxiliary traction systems are employed in portions of tunnels 20, 30, 40 having a grade over 2°.

As previously described and shown in FIGS. 1 and 3, operating tunnels 30, 40 are parallel, with upper tunnel 30 disposed above lower tunnel 40. In addition, as best shown in FIG. 5, upper tunnel 30 laterally overlaps with lower tunnel 40. More specifically, each tunnel 30, 40 has a central or longitudinal axis 35, 45, respectively, that lies in a vertical plane 36, 46, respectively. Plane 36 intersects tunnel 40, and plane 46 intersects tunnel 30, however, planes 36, 46 are laterally spaced apart by a horizontal distance D₃₆₋₄₆. Thus, lower tunnel 40 follows the same path as upper tunnel 30, but is laterally offset from upper tunnel 30. As a result, tunnels 30, 40 partially laterally overlap a horizontal distance D_(o) that is preferably between 5 and 20 ft. As previously described, the diameter D₃₀ of upper tunnel 30 is preferably 20-50 ft. Thus, the ratio of the horizontal distance D_(o) to the diameter D₃₀ of upper tunnel 30 is preferably between 0.1 and 1.0. Although tunnels 30, 40 are laterally offset in this embodiment, in other embodiments, the operating tunnels (e.g., tunnels 30, 40) may be laterally centered such that the central axis of each lies in a common vertical plane.

As previously described and shown in FIGS. 1 and 3, linear sections 31, 41 of operating tunnels 30, 40 are oriented perpendicular to linear sections 21 of access tunnel 20 in top view. Thus, although tunnel 20 does not intersect either tunnel 30, 40, sections 21 cross below and perpendicular to sections 31, 41. One connection 50 (see FIG. 2) between tunnel 20 and each tunnel 30, 40 is provided at each location where tunnel 20 crosses tunnel 30, 40, respectively, thereby providing numerous, periodic points to access operating tunnels 30, 40 from tunnel 20. As best shown in FIGS. 2 and 4, each connection tunnel 50 simultaneously slopes upward and curves to connect tunnel 20 with one tunnel 30, 40. At each location where tunnel 20 crosses tunnels 30, 40, connection tunnels 50 extending to tunnel 30, 40 intersect opposite lateral sides of tunnel 20. In this embodiment, each connection tunnel 50 is oriented at an angle α₅₀ (measured from horizontal, see FIG. 4) that is preferably between 3° and 10°. Because connection tunnels 50 have grades greater than 2°, a transition between rubber tired vehicles and rail cars is provided at the intersection of each tunnel 50 and tunnel 20, 30, 40.

A plurality of cut outs or storage caverns are preferably provided at various locations along the lateral sides of connection tunnel 50. Each such storage cavern preferably extends laterally between 30 and 150 ft. into formation 5 from connection tunnel 50, and further, such storage caverns are preferably located about every 100 to 500 ft. along connection tunnel 50. In general, the storage caverns can have any suitable size and geometry, but is preferably a uniform cross section with vertical and horizontal dimensions between 15 and 50 ft. The storage caverns provide storage space to accommodate equipment. For example, due to the limited size of tunnel 30, equipment may temporarily be moved into one or more caverns to allow other equipment to be moved through tunnel 30.

Referring now to FIGS. 2 and 4-6, a plurality of passages or conduits 60 extend vertically from upper operating tunnel 30 to lower operating tunnel 40 at drill site 15. As best shown in FIG. 6, each conduit 60 has a central or longitudinal axis 65, a first or upper end 60 a at the floor of upper tunnel 30, and a second or lower end 60 b at the ceiling of lower tunnel 40. Upper ends 60 a of each conduit 60 are preferably closed off or plugged (e.g., with a steel cap) to reduce the risk of equipment or personnel falling therein. Axes 65, and hence conduits 60, are vertical, parallel, and spaced apart a horizontal distance D₆₀₋₆₀ that is preferably between 25 and 100 ft. In addition, each conduit 60 has a diameter D₆₀ that is preferably between 20 and 36 in.

In this embodiment, each axis 65 intersects longitudinal axis 35 of upper tunnel 30 and lies in vertical plane 36. Thus, conduits 60 are generally arranged in a row along tunnels 30, 40. Because plane 36 is laterally offset from parallel vertical plane 46, axes 65 do not intersect axis 45. Rather, as best shown in FIG. 5, axes 65 intersect lower operating tunnel 40, but are laterally offset from axis 45. As will be described in more detail below, this arrangement permits wellbore pressure control equipment (e.g., blowout preventer) in lower operating tunnel 40 to be positioned proximal the sidewall of lower tunnel 40, thereby providing space in the central portion of lower tunnel 40 for other equipment such as mud control equipment, a crane, etc.

Referring again to FIGS. 2 and 4, system 10 also includes a plurality of vertical vents 70 that interconnect tunnels 20, 30, 40 to the surface 6 to provide ventilation and prevent the accumulation of noxious gases within tunnels 20, 30, 40. Vents 70 may be used to introduce fresh air or to exhaust air and may be located at any convenient location, as required by the design of the tunnel system 10 or the available access on surface 6. Vents 70 may interconnect with each tunnel 20, 30, 40 or only one of tunnel as required by the ventilation design. Each vent 70 is connected to the side of the tunnel 20, 30, 40 via a short alcove.

Referring now to FIG. 6, in this embodiment, a rail system 80 is provided in each tunnel 30, 40. Each rail system 80 includes a track 81 and a plurality of rail cars 82 that roll along the corresponding track 81. Each track 81 is disposed along the floor of the corresponding tunnel 30, 40, and further, extends the entire length of the corresponding tunnel 30, 40. Accordingly, rail system 80 may be described as a “floor” rail system. In general, rail systems 80 may comprise any suitable track or rail system known in the art such as those conventionally used in mining operations. Rail system 80 preferably operates on electrical power to reduce exhaust fumes associated with diesel-powered transport systems and vehicles.

Rail systems 80 can be used to transport equipment and/or personnel through system 10. For example, drilling rigs and mud circulation equipment, cranes for lifting heavy equipment, etc. can be transported through tunnels 30, 40 on rail cars 82.

In this embodiment, rail systems 80, 90 are not provided in tunnels 20, 50, due at least in part to the grades of tunnels 20, 50. Thus, as previously described, rubber-tired transport vehicles are employed in tunnels 20, 50. However, as appropriate in other embodiments, rail systems (e.g., rail system 80) can be used in the access tunnel(s) (e.g., tunnels 20) and connection tunnel(s) (e.g., tunnels 50).

Referring still to FIG. 6, system 10 also includes an additional rail system 90 in upper operating tunnel 30. Rail system 90 includes a track 91 mounted to the ceiling of upper tunnel 30 and a plurality of cassettes or carriages 92 moveably coupled thereto. In this embodiment, carriages 92 carry drilling pipe joints along track 91 through tunnel 30. Track 91 includes a first rail extending longitudinally along one lateral side of the ceiling of tunnel 30, a second rail parallel to the first rail and extending longitudinally along the opposite lateral side of the ceiling of tunnel 30, and a plurality of longitudinally spaced U-joint couplings extending between the rails. Accordingly, rail system 90 may be described as an “overhead” rail system. Carriages 92 can be moved along either rail of track 91, and moved between rails of track 91 via the U-joint couplings. It should be appreciated that the ability to move carriages 92 along either side of tunnel 30, as well as move carriages 92 between the sides of tunnel 30, enables carriages 92 and the equipment transported thereon to be moved around and between other equipment disposed in the middle of tunnel 30.

In general, tunnels 20, 30, 40, 50 are formed using tunneling practices known in the art. For example, tunnels 20, 30, 40, 50 can be dug or drilled using a road header or a tunnel boring machine. In addition, tunnels 20, 30, 40, 50 are preferably lined to enhance structural integrity and to provide insulation. For example, mesh or shotcrete may be used to line each tunnel 20, 30, 40, 50.

The tunnels 20, 30, 40 are constructed (i.e., formed and lined) concurrently with connector tunnels 50 added at intersections between the upper/lower tunnel 30, 40 and the access tunnel 20. Similarly, storage caverns are created during the excavation of the connector tunnels 50 using conventional drilling and/or boring techniques. Next, rail systems 80, 90 are installed in tunnels 30, 40 and equipment to be used in the tunnels 30, 40 is transported therein. Upon completion of the first connector tunnel 50, drilling may begin. Depending on the amount of ventilation available through the tunnels 20, 30, 40 to the surface, drilling may commence prior to the installation of a vent 70; however if ventilation is inadequate, drilling may commence after the installation of one or more vents 70.

Following the construction of tunnels 20, 30, 40, 50, conduits 60 are drilled between tunnels 30, 40. In general, conduits 60 may be drilled by any suitable drilling technique known in the art such as with the Orion in-the-hole drill available from Cubex® of Winnipeg, Canada. Further, conduits 60 may be drilled downward from upper tunnel 30 through the formation 5 to lower tunnel 40, or drilled upward from lower tunnel 40 through formation 5 to upper tunnel 30. During drilling operations in either tunnel 30, 40, the drilling rig preferably derives weight-on-bit (WOB) by bearing against the floor or ceiling of the corresponding tunnel 30, 40.

Referring now to FIGS. 1 and 2, in this embodiment, access tunnel 20 is used primarily to transport materials, equipment, and personnel to/from tunnels 30, 40, as well as for ventilation. Namely, materials, equipment, and personnel needed for drilling and/or production operations are transported through access tunnel 20 and connection tunnels 50 to the desired location in upper tunnel 30 or lower tunnel 40 using rail systems 80. When materials or equipment in tunnel 30, 40 inhibit the passage of other materials, equipment, or personnel, access tunnel 20 can be used to circumvent the impassable portions of tunnels 30, 40. Similarly, access tunnel 20 can be used as a short cut between different locations in tunnels 30, 40. For example, as shown in FIG. 3, the distance from tunnel access 11 to point A in tunnel 30 is shorter traveling through access tunnel 20 and connection tunnel 50 where tunnels 20, 30 cross at point A than traveling exclusively though the upper tunnel 30 from access 11 to point A. In addition, access tunnel 20 can serve as a temporary storage location when rearranging or reordering equipment or materials in tunnels 30, 40.

Referring now to FIGS. 5 and 6, upper operating tunnel 30 is used primarily to house an automated modular underground drilling rig 100 at site 15 for drilling a plurality of wellbores 61 extending downward from lower tunnel 40. Tunnel 30 also houses tubulars used during drilling operations (e.g., drill pipe, coiled tubing, etc.). For example, carriages 92 store drilling pipe joints 96, deliver drilling pipe joints 96 to rig 100 during drilling operations, and carry pipe joints 96 from rig 100 during tripping operations. Although rig 100 is preferably automated, on occasion, operating personnel may be present within upper operating tunnel 30.

Lower tunnel 40 is used primarily to house a blowout preventer (BOP) 43 and drilling mud circulation equipment 44 at drill site 15. In general, BOP 43 can be any suitable BOP known in the art. The physical separation of drilling rig 100 in upper tunnel 30 from BOP 43 and mud circulation equipment 44 in lower tunnel 40 provides a few potential advantages. In particular, BOP 43 and mud circulation equipment 44 operate autonomously for the most part, thereby reducing the need for personnel to be present in lower operating tunnel 40. This offers the potential to reduce safety risks to personnel resulting from a blowout. Further, in the event of a blowout or leak, containment is provided by lower tunnel 40, generally away from personnel. In addition, during production operations subsequent to drilling operations, the production equipment (e.g., pumps, pipelines, etc.) are preferably disposed in tunnel 40. This allows the physical separation of hydrocarbons from personnel in upper tunnel 30.

Referring now to FIGS. 6-8, in this embodiment, drilling rig 100 is a specially designed fit-for-purpose rig including a base assembly 120, a drilling assembly 140 mounted to base assembly 120, an upper frame assembly 160 positioned atop drilling assembly 140, a bearing or support hood 220 supported by upper frame assembly 160, and a pipe handling assembly 190 positioned adjacent the drilling assembly 140. Bearing hood 220 is braced against the ceiling of upper tunnel 30 during drilling operations.

Base assembly 120 generally supports rig 100 on track 81 of upper tunnel 30 and includes a car 82, a rig floor 124 pivotally coupled to car 82, a plurality of positioning assemblies 130 positioned about the perimeter of car 82, and a clamping system 114 mounted to rig floor 124. Car 82 is provided with a wheels or rollers 122 to facilitate the movement of rig 100 through tunnel 30 along track 81. A positioning assembly 130 is disposed at each corner of car 82 and functions to secure and maintain the position of drilling rig 100 within tunnel 30 during drilling and tripping operations. Clamping system 114 is attached to drilling rig floor 124 and is generally disposed about a drilling hole in base assembly 120. Clamping assembly 114 aids in making and breaking threaded connections during drilling and tripping operations. Examples of clamping systems suitable for use with drilling rig 100 are disclosed in U.S. Patent Application Ser. No. 61/783,859, which is hereby incorporated herein by reference in its entirety.

Drilling assembly 140 is mounted to base assembly 120 and provides rotational torque to the drill string and WOB during drilling operations. Upper frame assembly 160 functions as an upper support and bracing structure for the modular underground drilling rig 100 and serves to interconnect rails of track 91 with rig 100 for the delivery and removal of pipe joints 96 via carriages 92. Pipe handling assembly 190 transfers pipe joints 96 between carriages 92 and rig 100, as well as initiates the threading/unthreading of pipe joints 96 to the upper end of the drill string during drilling and tripping operations. Examples of automated modular drilling rigs suitable for use in system 10 are disclosed in U.S. Patent Application Ser. No. 61/784,199, which is hereby incorporated herein by reference in its entirety.

During drilling operations, a drillstring supported by rig 100 is lowered downward through conduit 60 into lower tunnel 40, and through BOP 43. With the drillstring extending through BOP 43, rig 100 drills downward from the floor of lower tunnel 40 into the formation 5 below, thereby forming wellbores 61.

In embodiments described herein, a closed loop drilling fluids circulation and management system is preferably employed during drilling operations. An exemplary embodiment of a closed loop drilling system 200 is shown in FIG. 9. System 200 generally circulates drilling fluid between a local drilling mud circulation system 230 disposed in lower tunnel 40 at drill site 15 and a central drilling fluid processing facility 300 located remote from drill site 15 at the surface 6 or storage cavern proximal a vent 70.

Central processing facility 300 includes a variety of components for processing used drilling fluid and converting it into clean drilling fluid. For example, central processing facility can include equipment including, without limitation, a degasser for removing gases from the drilling fluid, solids separation equipment for removing solids from drilling fluid, and a drilling fluid transfer pump for facilitating the flow of drilling fluid through facility 300. Although only one drill site 15 in system 10 is shown in FIG. 9, it should be appreciated that other drill sites in system 10 (or outside system 10) can be coupled to central processing facility 300.

Central processing facility 300 supplies clean, processed drilling fluid to local mud circulation system 230 via a primary supply line 280. Local drilling mud circulation system 230 (including equipment 44) pumps the clean, processed drilling fluid to rig 100. The clean, processed drilling fluid is pumped down the drillstring, through the face of the drill bit, and returns to BOP 43 via the annulus between the drillstring and the sidewall of wellbore 61. While being circulated through wellbore 61, solids (e.g., formation cuttings), liquids (e.g., hydrocarbons, water, etc.), gases (e.g., hydrogen sulfide, natural gas, etc.), or combinations thereof become entrained in the drilling fluid, thereby transitioning clean drilling fluid into used drilling fluid. The dirty, used drilling fluid from the annulus is supplied back to local mud circulation system 230 via a rotating head on BOP 43. The returned drilling fluid is partially processed by local mud circulation system 230 to condition large solids, and then pumped back to central processing facility 300 via a primary return line 285 for further processing and conditioning. The local mud circulation system 230 is a pressurized, sealed and automated system. Examples of closed loop drilling fluid circulation and management systems that can be used with system 10 are described in U.S. Patent Application Ser. No. 61/783,979, which is hereby incorporated herein by reference in its entirety.

Referring now to FIGS. 2 and 6, the operation of system 10 to recover hydrocarbons (e.g., conventional and/or heavy oil) in formation 5 is schematically shown. In this embodiment, a plurality of wellbores 61 are drilled into the formation 5 from lower tunnel 40 to access the hydrocarbon-bearing portion of formation 5 therebelow. In particular, drilling rig 100 is disposed on car 82 in upper operating tunnel 30, and thus, can be moved through tunnel 30 and positioned over conduits 60. With rig 100 disposed over a selected conduit 60, a drillstring supported by rig 100 is lowered downward through conduit 60 into lower tunnel 40, and through BOP 43. With the drillstring extending through BOP 43, rig 100 drills downward from the floor of lower tunnel 40 into the formation 5 below, thereby forming wellbores 61. Thus, each wellbore 61 is coaxially aligned with a corresponding conduit 60, and during drilling operations, each wellbore 61 is coaxially aligned with one BOP 43. Accordingly, the upper portions of wellbores 61 are parallel, arranged side-by-side in a row extending along lower tunnel 40, and have the same spacing as conduits 60. In this embodiment, the close proximity and high density of wellbores 61 is particularly preferred to access and produce heavy oil (e.g., bitumen) in the hydrocarbon-bearing portion of formation 5. It should be appreciated that as wellbores 61 extend into formation 5, their paths may deviate, change direction, etc. For example, one or more wellbores 61 may be drilled horizontally through the reservoir.

Following the drilling of a particular wellbore 61, rig 100 is moved with car 82 to the adjacent conduit 60, and BOP 43 or other BOP 43 is moved with a crane disposed on a rail car 82 in lower tunnel 40 into alignment with the adjacent conduit 60, thereby enabling rig 100 to drill an adjacent wellbore 61. This process is repeated to form the line of wellbores 61 extending downward from lower tunnel 40 into formation 5 therebelow.

In the manner described, embodiments of system 10 provides a means to exploit and develop hydrocarbon reservoirs in geographic regions where conventional surface drilling and/or production is not practical due to environmental conditions (e.g., harsh weather), environmental sensitivity, government regulations, costs, etc. In particular, because wellbores 61 are drilled downward from lower tunnel 40 and do not extend to the surface 6, the footprint of system 10 at the surface and associated environmental impacts are significantly reduced. Thus, system 10 allows for the recovery of hydrocarbons in harsh environments, environmentally sensitive areas or areas where the surface footprint is an issue. Further, by drilling from underground tunnels 30, 40, operations can continue drilling unabated in harsh weather locations such as those seen in the arctic.

Although system 10 has been described primarily in the context of hydrocarbon drilling operations, system 10 can also be used in production operations. Namely, after drilling wellbores 61, each wellbore 61 is cased and/or lined, and a production tree is mounted to a wellhead attached to the upper end of the casing in lower tunnel 40. Hydrocarbons are then produced through wellbores 61 to the corresponding trees in lower tunnel 40 and routed to underground or surface storage vessels and/or transported to other locations as desired.

While preferred embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the systems, apparatus, and processes described herein are possible and are within the scope of the invention. For example, the relative dimensions of various parts, the materials from which the various parts are made, and other parameters can be varied. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. Unless expressly stated otherwise, the steps in a method claim may be performed in any order. The recitation of identifiers such as (a), (b), (c) or (1), (2), (3) before steps in a method claim are not intended to and do not specify a particular order to the steps, but rather are used to simplify subsequent reference to such steps. 

What is claimed is:
 1. A system for recovering hydrocarbons from a reservoir contained in a subterranean formation, comprising: an upper tunnel extending through the formation; a lower tunnel extending through the formation below a portion of the upper tunnel; a plurality of conduits extending from the upper tunnel through the formation to the lower tunnel; and a plurality of wellbores extending from the lower tunnel to the reservoir in the formation.
 2. The system of claim 1, further comprising: an access tunnel extending through the formation; and a connection tunnel connecting the access tunnel to the upper or lower tunnel.
 3. The system of claim 1, wherein each wellbore is coaxially aligned with one conduit.
 4. The system of claim 3, wherein the conduits are vertically oriented, parallel, and arranged in a row.
 5. The system of claim 2, further comprising: a drilling rig disposed in the upper tunnel over a first of the conduits; and a BOP disposed in the lower tunnel over one of the wellbores that is coaxially aligned with the first of the conduits.
 6. The system of claim 5, further comprising: a first rail system disposed along a floor of the upper tunnel or the lower tunnel; and a second rail system coupled to a ceiling of the upper tunnel.
 7. The system of claim 6, wherein the drilling rig is disposed on a rail car of the first rail system.
 8. The system of claim 1, wherein the lower tunnel extends parallel to the portion of the upper tunnel.
 9. The system of claim 8, wherein the upper tunnel has a longitudinal axis and the lower tunnel has a longitudinal axis, and wherein the longitudinal axis of the upper tunnel lies in a first vertical plane and the longitudinal axis of the lower tunnel lies in a second vertical plane; wherein the second vertical plane intersects the portion of the upper tunnel; wherein the second vertical plane is laterally spaced from the first vertical tunnel along the portion of the upper tunnel.
 10. The system of claim 1, wherein the upper tunnel has a diameter D₁ and the lower tunnel has a diameter D₂; wherein the ratio of D₂ to D₁ is between 0.3 and 1.0.
 11. The system of claim 10, wherein the upper tunnel laterally overlaps the lower tunnel by a horizontal distance D_(o); wherein the ratio of distance D_(o) to the diameter D₁ is between 0.1 and 1.0.
 12. A method for recovering hydrocarbons from a reservoir in a subterranean formation, the method comprising: constructing an upper tunnel that extends through the formation; constructing a lower tunnel that extends through the formation and is disposed below a portion of the upper tunnel; and drilling downward from the upper tunnel through the lower tunnel and into the formation toward the reservoir.
 13. The method of claim 12, further comprising: producing hydrocarbons in the reservoir through the formation to the lower tunnel.
 14. The method of claim 12, further comprising: constructing an access tunnel that extends through the formation; and constructing at least one connection tunnel connecting the access tunnel to the upper or lower tunnel.
 15. The method of claim 14, further comprising: positioning a drilling rig in the upper tunnel over a first drilling location; positioning a BOP in the lower tunnel over the first drilling location; and advancing a drillstring from the drill rig through the BOP to drill at the first drilling location.
 16. The method of claim 15, further comprising: moving the drilling rig through the upper tunnel to a second drilling location; moving the BOP in the lower tunnel to the second drilling location; and advancing a drillstring from the drill rig through the BOP to drill at the second drilling location.
 17. The method of claim 12, wherein the lower tunnel is parallel to the upper tunnel.
 18. A method for recovering hydrocarbons from a reservoir in a subterranean formation, the method comprising: (a) positioning a drilling rig at a first location in an upper tunnel traversing the formation; (b) positioning a BOP at a first location in the lower tunnel that is below the first location in the upper tunnel; (c) advancing a drillstring from the drilling rig through the BOP; and (d) drilling a first wellbore downward from the lower tunnel with the drilling rig and drillstring.
 19. The method of claim 18, further comprising: (e) moving the drilling rig to a second location in the upper tunnel; (f) moving the BOP to a second location in the lower tunnel that is below the second location in the upper tunnel; and (g) advancing a drillstring from the drilling rig through the BOP; and (h) drilling a second wellbore downward from the lower tunnel with the drilling rig and drillstring.
 20. The method of claim 18, wherein (a) comprises transporting the drilling rig to the first location in the upper tunnel with one or more rail cars, and wherein (b) comprises transporting the BOP to the first location in the lower tunnel with a rail car.
 21. The method of claim 18, wherein (a) comprises moving the drilling rig through an access tunnel connected to the upper tunnel; and wherein (b) comprises moving the BOP through the access tunnel connected to the lower tunnel.
 22. The method of claim 18, further comprising: (e) producing hydrocarbons in the reservoir through the formation and the first wellbore. 